One of the biggest problems caused from vehicles using fossil fuel, such as gasoline and diesel oil, is creation of air pollution. A technology of using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has been attempted.
High output and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). To this end, a plurality of small-sized secondary batteries (unit cells) are connected in series or in parallel with each other so as to constitute a battery module, and a plurality of battery modules are connected in parallel or in series with each other so as to constitute a battery pack.
In such a high-output, large-capacity secondary battery, however, a large amount of heat is generated from the unit cells during the charge and the discharge of the unit cells. When the heat generated from the unit cells during the charge and the discharge of the unit cells is not effectively removed, heat is accumulated in the unit cells with the result that the unit cells are degraded. Consequently, it is necessary to provide a cooling system for cooling a battery pack, which is a high-output, large-capacity secondary battery.
A principle of a cooling system for cooling a battery pack, which is generally used, will be described with reference to FIG. 1.
Referring to FIG. 1, a battery pack cooling system 1 includes a battery pack 2 comprising a plurality of batteries, a coolant inlet part 3 mounted at the lower end surface of the battery pack 2, and a coolant outlet part 4 mounted at the upper end surface of the battery pack 2. The battery pack 2 comprises a plurality of battery modules 5. Each battery module 5 comprises a plurality of unit cells 6, which are electrically connected with each other. Between the unit cells of each battery module 5 are formed small gaps, through which coolant flows. Consequently, coolant introduced through the coolant inlet part 3 flows through the gaps to remove heat generated from the unit cells 6, and is then discharged through the coolant outlet part 4 mounted at the upper end surface of the battery pack 2.
However, the battery pack cooling system 1 constructed as shown in FIG. 1 has the following problems.
First, since the coolant inlet part 3 and the coolant outlet part 4 are mounted at the lower end surface and the upper end surface of the battery pack 2, respectively, it is necessary to provide spaces for installing such coolant guiding members at the lower end surface and the upper end surface of the battery pack 2. As a result, the total size of a battery system is increased, which is not preferable.
Second, it is difficult for the coolant introduced through the coolant inlet part 3 to be uniformly supplied to the respective battery modules 5, and therefore, the temperature difference between the unit cells 6 may greatly increase. Recent research shows that the great temperature difference between the unit cells 6 is one of principal factors greatly decreasing the performance of the battery pack 2.
In order to solve the above-mentioned problems, especially, the volume of the battery system, there have been proposed technologies for providing a coolant inlet port and a coolant outlet port of the cooling system at one side of the battery system to reduce the volume of the battery system, which are disclosed in Japanese Unexamined Patent Publication No. 2004-152769, Japanese Unexamined Patent Publication No. 2004-087368, and Japanese Unexamined Patent Publication No. 2004-001683. However, a technology for providing the coolant inlet port and the coolant outlet port at one side of the battery system using the structure of a vehicle to minimize the volume of the battery system has not been proposed yet.
Also, simple partitions may be mounted in the battery pack to guide the passage of coolant, thereby reducing the temperature difference between the unit cells, as disclosed in Japanese Unexamined Patent Publication No. 2004-152769. However, the partitions cannot effectively reduce the temperature difference between the unit cells, and furthermore, the coolant may not be supplied due to the partitions.